The invention relates to a method for current conditioning, comprising                transporting a primary current through a primary coil,        coupling a secondary coil to the primary coil via a common magnetic flux, wherein the secondary coil comprises a superconductor capable of quenching, with the quenching causing a transition of the superconductor from a low resistance superconducting state to a high resistance quenched state,        and in the low resistance superconducting state of the secondary coil, guiding a major fraction of the common magnetic flux of the primary coil and the secondary coil within a ferromagnetic medium.        
Such a method is known from DE 195 24 579 A1.
The present invention relates to a method for current conditioning and may be employed, in particular, in the fields of energy generation, transmission and distribution for grid and equipment protection against fault current.
Current conditioning is necessary in generation, transmission and distribution of energy. Current conditioning considered as current limitation of fault currents (e.g. short circuited currents) allows not only to protect electrical equipment against over-currents, but also to exploit already available equipment at a higher level of transported power.
In general, current conditioning may be provided via numerous methods, for instance, using methods of magnetic saturation (saturation reactors), mechanical switching, electronic switching (based on solid state switches), etc. Recently developed methods of current conditioning are based on the transition of a superconductor from a low resistive (superconducting) state to a high resistive (quenched) state and allow a conditioning of high currents at medium and high voltages. The most economic current conditioning methods using the transition of a superconductor, in particular with respect to power losses and cryo-consumption, are based on an inductive coupling of a circuit current to be conditioned with a superconductor. Here, no current leads connecting ambient temperature areas with cryogenic temperature areas are necessary; the superconductor (typically a short-circuited superconducting coil) may be completely encapsulated, so thermal insulation is simple and efficient. Several current conditioning methods aimed for limitation of fault currents were developed in the later field.
JP 04 112 620 A discloses a method for current conditioning wherein a circuit current (primary current) to be conditioned is transported through a primary coil, and a secondary coil, arranged radially within the primary coil and made of a superconducting material, is coupled to the first coil through a common magnetic flux. Both in the low resistance (superconducting) state and the high resistance (quenched) state of the secondary coil, the common magnetic flux is guided in air. The method allows only a small current conditioning effect, and more specifically, only a minor effect of current limitation.
A better degree of current conditioning was achieved when the guiding of the magnetic flux was performed within a ferromagnetic medium, compare the method described in DE 19 524 579. Here the circuit current (primary current) is transported through a primary coil, and a secondary coil, arranged radially outside the primary coil and made of a superconducting material, is coupled to the first coil through a common magnetic flux. Both in the high resistance (quenched) state and the low resistance (superconducting) state of the secondary coil, the common magnetic flux is guided within a ferromagnetic transformer core. However, in the high resistance state of the secondary coil, this method of current conditioning introduces severe harmonic distortions in the primary current; the total harmonic distortions may easily exceed 20 to 30%. These harmonic distortions are a severe threat to the safety of electric grids and electric equipment supplied with the primary current.
It is the object of present invention to provide an economic and efficient method for current conditioning which reduces harmonic distortions.